Cold rolled steel sheet having excellent corrosion resistant to sulfuric acid

ABSTRACT

A cold rolled steel sheet which has an excellent corrosion resistance to sulfuric acid, the steel includes in weight %: 0.15% or less of C, 1.0% or less of Si, 0.2˜1.5% of Mn, 0.03% or less of S, 0.03% or less of P, 0.01˜0.1% of Al, 0.2˜1.0% of Cu, 0.02˜0.2% of Co, and a balance of Fe and other unavoidable impurities.

FIELD OF THE INVENTION

[0001] The present invention relates to a cold rolled steel sheet whichhas an excellent corrosion resistance to sulfuric acid. Morespecifically, the present invention relates to a cold rolled steel sheethaving an excellent corrosion resistance to sulfuric acid, which is usedas the material of the components exposed to sulfuric acid like thepre-heater of a thermoelectric power plant, boiler components and thelike.

BACKGROUND OF THE INVENTION

[0002] Generally, it is known that a large mount of Cu is added in thesulfuric acid corrosion resistant steel so that the corrosion of thesulfuric-acid-resistant steel can be slowed down under the sulfuric acidatmosphere. However, although Cu is superior in slowing down thesulfuric acid corrosion compared with other additives, if it is too muchadded, there occur problems such as the generation of cracks and thelike during the hot rolling.

[0003] Japanese Patent Application Laid-open No Hei-9-25536 discloses anacid-corrosion resistant steel which aims at solving the above describedproblem. This steel includes in weight %: 0.01˜0.15% of C, 0.1˜0.5% ofSi, 0.1˜0.5% of Mn, 0.03% or less of P, 0.01% or less of S, 0.2˜1.0% ofCu, 0.5% or less of Ni, 2.0% or less of Cr, 0.1% or less of Al,0.01˜1.0% of Sn and/or Sb, 0.005% or less of B, 0.2% or less of V, Nb:0.2% or less of Nb, and 0.02% or less of Ti.

[0004] Another example is Japanese Patent Application Laid-open NoHei-10-110237 which includes in weight %: 0.01˜0.15% of C, 0.1˜0.5% ofSi, 0.1˜0.5% of Mn, 0.03% or less of P, 0.005% or less of S, 0.2˜1.0% ofCu, 0.5% or less of Ni, 2.0% or less of Cr, 0.1% or less of Al, 0.2% orless of V, 0.2% or less of Nb, 0.2% or less of Ti, one selected fromamong 0.01˜1.0% of Sn and/or Sb, 0.001˜0.01% of B, and 0.01˜0.5% of Mo,and a balance of Fe and other unavoidable impurities.

[0005] That is, in the above described invention, a proper amount of Cuis added and at the same time, other elements are compositely added.Thus a superior acid-corrosion resistance is aimed to be achieved, butat a low temperature and a low sulfuric acid concentration, thecorrosion rate is still high, this being the problem.

SUMMARY OF THE INVENTION

[0006] In order to solve the above described problems, the presentinventors carried out research and experiments repeatedly, and theresult is the present invention.

[0007] Therefore it is an object of the present invention to provide acold rolled steel sheet with an excellent sulfuric acid corrosionresistance, in which not only Cu but also a proper amount of Co is addedso that the corrosion rate can be greatly lowered at a lowtemperature-low sulfuric acid concentration, thereby providing a coldrolled steel having a superior corrosion resistance against sulfuricacid.

[0008] In achieving the above object, the cold rolled steel sheet withan excellent sulfuric acid corrosion resistance according to the presentinvention includes in weight %: 0.15% or less of C, 1.0% or less of Si,0.2˜1.5% of Mn, 0.03% or less of S, 0.03% or less of P, 0.01˜0.1% of Al,0.2˜1.0% of Cu, 0.02˜0.2% of Co, and a balance of Fe and otherunavoidable impurities.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0009] Now the present invention will be described in detail.

[0010] The cold rolled steel sheet with an excellent sulfuric acidcorrosion resistance according to the present invention includes inweight %: 0.15% or less of C, 1.0% or less of Si, 0.2˜1.5% of Mn, 0.03%or less of S, 0.03% or less of P, 0.01˜0.1% of Al, 0.2˜1.0% of Cu,0.02˜0.2% of Co, and a balance of Fe and other unavoidable impurities.

[0011] If the content of C exceeds 0.15%, the weldability is severelylowered, and therefore, defects are liable to occur. Accordingly, thecontent of C should be preferably limited to 0.15% or less.

[0012] The element Si is added mainly to improve the strength of thesteel, but if its content exceeds 1.0%, then the corrosion resistance isgreatly aggravated at the low temperature-low sulfuric acidconcentration. Accordingly, the content of Si should be preferablylimited to 1.0% or less.

[0013] The element Mn is usually added to prevent the hot shortnesscaused by the solid solute sulfur which has been precipitated in a formof manganese sulfide. In the present invention, Mn is added not only forpreventing the hot shortness, but also to improve the strength of thesteel.

[0014] In the present invention, the content of Mn should be preferablylimited to 0.2˜1.5%. The reason is as follows. That is, if the contentof n exceeds 1.5%, then not only the strength improvement effect islowered, but also the sulfuric acid corrosion resistance is slightlyaggravated. On the other hand, if the content of Mn is less than 0.2%,the hot shortness is likely to occur.

[0015] The content of S should be preferably as low as possible in thepresent invention. If its content exceeds 0.03%, then defects are liableto occur due to the hot rolling brittleness, and therefore, its upperlimited should be preferably set to 0.03%.

[0016] The content of P should also be preferably as low as possible. Ifits content exceeds 0.03%, the corrosion resistance is greatly lowered,and therefore its upper limit should be preferably limited to 0.03%.

[0017] The element Al is added as a deoxidizing agent. If its content isless than 0.01%, then the deoxidizing effect is too meager, while if itscontent exceeds 0.1%, then the surface defects are apt to occur due tothe increase of Al oxides. Accordingly, the content of Al should bepreferably limited to 0.01%-0.1%.

[0018] The element Cu is added for improving the sulfuric acid corrosionresistance. However, its effects are significant lowered only if itscontent is below 0.2%. However, if its content exceeds 1.0%, theadditional effects are negligible compared with the added amount.Accordingly the content of Cu should be preferably limited to 0.2-1.0%.

[0019] The addition of Co is the characteristic feature of the presentinvention, and Co greatly improves the corrosion resistance at the lowtemperature - low sulfuric acid concentration.

[0020] In the present invention, the content of Co should be preferablylimited to 0.02˜0.2%, and the reason is as follows. That is, if thecontent of Co is less than 0.02%, then its addition effects are toonegligible, while if its content exceeds 0.2%, the improvement of thecorrosion resistance is not sufficient in view of the addition amount.

[0021] If the economy is taken into account, the content of Co should bemore preferably limited to 0.02˜0.15%.

[0022] Further, in the present invention, the contents of Cu and Co haveto be respectively controlled, and the atomic ratio of Cu/Co also has tobe properly controlled, so that a superior corrosion resistance can beensured. That is, the atomic ratio of Cu/Co should be preferably limitedto 2.0 or more. If the mentioned atomic ratio is less than 2.0, then theelution of Cu within the steel is decreased, with the result that thecorrosion resistance is aggravated.

[0023] Meanwhile, the steel which is composed as described above ishot-rolled and cold-rolled, and then a continuous annealing or a batchannealing is carried out by maintaining the steel above therecrystallization temperature for 10 seconds or more, thereby obtaininga cold rolled steel sheet with a superior corrosion resistance.

[0024] Now the present invention will be described based on an actualexample.

EXAMPLE

[0025] Steel ingots having the compositions of Table 1 below wereprepared.

[0026] Then all the ingots were maintained at a temperature of 1250° C.for 1 hour, and then a hot rolling was carried out. Under thiscondition, the hot finish rolling was started at 1100° C., the coilingtemperature was 650° C., and the final thickness was 4.5 mm.

[0027] Then the hot rolled test pieces were pickled so as to remove thesurface oxide films, and then a 73%-cold-rolling was carried out down toa final thickness of 1.2 mm. Then a degreasing and a continuousannealing were carried out. Under this condition, the continuousannealing was carried out at a temperature of 830° C. for 30 seconds.

[0028] Then, for these annealed test pieces, the sulfuric acid corrosionresistance at the low temperature-low concentration conditions wasmeasured in the following manner. That is, the test pieces were immersedin a 50%- sulfuric acid solution at a temperature of 70° C. for onehour, and then, the corrosion losses were measured, and the measuredresults are shown in Table 1 below. TABLE 1 Cu/Co Corrosion Chemicalcomposition (Weight %) Atomic loss Classfctn C Si Mn P S Al Cu Co ratio(mg/cm² · hr) Invntv 0.08 0.1 0.5 0.01 0.02 0.04 0.35 0.06 5.41 5.3steel 1 Invntv 0.084 0.1 0.8 0.009 0.023 0.05 0.25 0.1 2.32 5.9 2 Invntv3 0.078 0.08 1.0 0.015 0.03 0.045 0.38 0.08 4.4 5.4 Invntv 4 0.081 0.090.3 0.012 0.018 0.035 0.40 0.15 2.47 4.9 Invntv 5 0.080 0.2 0.8 0.0200.018 0.035 0.28 0.15 1.73 10.8 Invntv 6 0.075 0.14 0.82 0.018 0.0150.032 0.22 0.18 1.13 12.5 Invntv 7 0.083 0.18 1.2 0.012 0.021 0.042 0.300.18 1.54 8.8 Cmprtv 0.080 0.1 0.5 0.010 0.012 0.04 0.15 0 — 58.6 steel1 Cmprtv 0.085 0.12 0.8 0.082 0.01 0.038 0.35 0.08 4.06 62.5 2 Cmprtv0.081 0.5 1.5 0.025 0.015 0.042 0.25 0 — 35.2 3 Cmprtv 0.079 0.15 1.00.02 0.02 0.04 0.20 0 — 24.5 4 Cmprtv 0.075 0.15 0.92 0.025 0.016 0.0350.26 0.004 60.25 23.5 5

[0029] As shown in Table 1 above, in all the cases of the inventivesteels (1˜7) in which not only Cu but also a proper amount of Co wereadded, there were shown low corrosion losses, thereby proving thesuperior sulfuric acid corrosion resistance. Particularly, not only thecontents of Cu and Co are appropriated, but also the atomic ratio ofCu/Co was controlled to 2 or more in the inventive steels (1˜4), andtherefore, these cases showed very low corrosion losses as low as 6.0mg/cm²·hr, thereby proving the superior sulfuric acid corrosionresistance.

[0030] On the other hand, in the cases of the comparative steels (1˜5)in which the chemical compositions departed from the range of thepresent invention, there were shown very much higher corrosion lossescompared with the inventive steels as shown in Table 1.

[0031] According to the present invention as described above, thesulfuric acid corrosion resistance is very much improved, and therefore,the life expectancy can be greatly improved in the pre-heaters ofthermoelectric power plants (in which the dew point corrosion occurs),in the pipes of boilers and the like.

What is claimed is:
 1. A cold rolled steel sheet with an excellentsulfuric acid corrosion resistance, comprising in weight %: 0.15% orless of C, 1.0% or less of Si, 0.2˜1.5% of Mn, 0.03% or less of S, 0.03%or less of P, 0.01˜0.1% of Al, 0.2˜1.0% of Cu, 0.02˜0.2% of Co, and abalance of Fe and other unavoidable impurities.
 2. The cold rolled steelsheet as claimed in claim 1, wherein its atomic ratio of Cu/Co iscontrolled to 2.0 or more.